1. Field of Invention
This invention relates to electrographic printing. Particularly, it teaches a method for improving gradations within the optical density range achievable by single-bit electrostatic printers.
2. Prior Art--Single-Bit Electrographic Printing Employing Four Primary Colors--FIG. 1
Electrographic printers generally apply an electrical charge pattern, or image, to a surface. This charge image is initially invisible. It is "developed" or made visible through the application of "toner." Toner consists of a collection of minute, typically micron-sized, electrically charged particles. The electrical charge on these particles is opposite to that in the charge pattern. Therefore the particles adhere to the charge pattern, but not to uncharged areas, rendering the charge image visible. Toner is available in a dry, powder form or a liquid form.
A typical electrographic printer in widespread use today is the 8900-Series electrostatic printer manufactured and sold by Xerox ColorgrafX Systems, Inc., 5853 Rue Ferrari, San Jose, Calif. 95138, U.S.A. These printers deposit a latent (undeveloped) electrical charge image on electrographic paper or other medium which is later "developed" using liquid toners, also called "inks." The final graphic print comprises an assemblage of binary dots, i.e. dots which are either present or absent. Another term used to describe this type of printing is "single-bit printing" since a single binary bit is either a one (dot present) or a zero (dot absent). In color images, the dots are printed successively in four primary color "planes." These planes are superposed as shown in FIG. 1 to form a single image. The black portion, plane, or layer 100 of the image, comprising dots 110, is generally laid down first. It is typically followed by a cyan layer 120, a magenta layer 130, and a yellow layer 140. Gray scale capability has been demonstrated but has generally eluded practical application. Images printed in four passes achieve the appearance of gray scale by varying the spacing among the dots using "dither patterns," in well-known fashion. In four-pass printing, the full dynamic range of each primary color is contained in its respective plane. The resulting printed image is excessively high in contrast. Shadows reproduce well. Faithful reproduction of mid-tones and highlights is not possible because of the limited dynamic range which can be accomplished using only dither patterns. Also, the number of colors which can be printed is limited since these colors are derived from dithered patterns of only four primary colors.
Prior Art--Single-Bit Electrographic Printing Employing Seven or Eight Colors--FIG. 2
An attempt has been made to improve the quality of single-bit printers--both electrostatic and inkjet--by printing seven or eight successive color passes instead of the previous four. A black pass 100' (FIG. 2) is typically printed first. In some cases, a second gray pass 200 is then printed. A true cyan pass 120' follows. Then a lighter blue or cyan pass 210 is printed. Next, a true magenta pass 130' is printed. This is followed by a lighter, magenta-like color pass 220. Finally a normal yellow pass 140' and a lighter, yellow-like pass 230 are printed. This seven-pass or eight-pass printing results in substantially improved images. However it also requires three or four extra ink colors, and the apparatus to apply them.
Prior Art--Single-Bit Electrostatic Printers--FIGS. 3 and 4
Single-bit electrostatic printers of the type made by Xerox ColorgrafX Systems employ a "contrast" adjustment. From the printer's controls it is possible to set the contrast (or saturation) of each color plane to values which lie between 0 and 99. When the contrast setting is "0" (FIG. 3), the dots 300 in a plane are absent. When the setting is 10, dots 310 are very faint. When the setting is 100, dots 330 are fully saturated. Normal settings lie in the 20 to 80 range. Dot 320 is representative of a contrast setting of 50. The contrast setting is applied equally to all dots in each color plane. Each color plane can have a different contrast setting. It is possible to set all contrast settings to greater or lesser levels in order to obtain lighter or darker prints. However these settings must be properly matched with each other to ensure proper color rendition of the final image. For example, if the cyan contrast setting is 100 and the yellow contrast setting is 0, it will not be possible to print any shade of green--which is the normal result of mixing of these two primary colors. Normally the contrast settings are adjusted so that cyan, magenta, and yellow contribute equal reflective optical densities in a three-color black image. Note that although the contrast control can be used to lighten or darken dots in a given plane, these dots are still "binary" in nature--they are either present or absent.
A prior-art, single-bit printing system is shown schematically in FIG. 4. A computer 400 contains a graphics program, a Raster Image Processor (RIP) program (described infra), and a print-sending program. In some systems, more than one computer is used. A first computer may contain the graphics program. A second may contain the RIP, and a third may contain the print-sending program. Files are easily transferred from one computer to another. Distributing the computing work can result in a saving of time. A typical print-sending program is manufactured and sold by Visual Edge Technology, Inc., 306 Potrero Ave., Sunnyvale, Calif. 94086-4113 U.S.A. The graphics program causes the image of interest 410 to be shown on monitor 420). The data which comprise image 410 are held in the computer's memory for use by the RIP program. On command, the RIP program converts the image from Red-Green-Blue (RGB) format to Cyan-Magenta-Yellow-blacK (CMYK) format and organizes it into color planes, described infra. The print sending program then sends the output of the RIP program to the printer, one color plane at a time in well-known fashion. Printer 430 then prints image 440. The density (or saturation) of color planes of image 440 are controlled by contrast controls 450. Images printed by single-bit printers using only four color planes are excessively high in contrast. Shadows print reliably but highlights and mid-tones do not reproduce faithfully. This is because the full dynamic range of a printed image must be accomplished using dither patterns. Instead of being printed with light-colored dots, light areas in an image using prior art technology comprise dark dots which are widely spaced. This is objectionable in most images.
Prior Art--Raster Image Processing
A color image which is to be printed is normally viewed on a computer screen which emits light. This image comprises an admixture of red, blue, and green dots of varying intensities. Since it is an admixture of colors, this is said to be an "additive, RGB" color image. Images printed on paper or other media generally comprise layers of pigments or dyes. These pigments or dyes overlay one-another and absorb, rather than emit, light. The paper image is thus said to be a "subtractive, CMYK" color image. In the present case, electrographic printers print the image in raster fashion. Each color plane image is printed in raster lines of one color at a time, starting it one end of the image and finishing at the other end, in well-known fashion. The second color is laid over the first, the third over the first and second, and so on.
Computer software, commonly called a RIP, is required to convert the image from an RGB image to a CMYK image. RIP software is manufactured and sold by Visual Edge Technology, Inc., 306 Potrero Ave., Sunnyvale, Calif. 94086-4113 U.S.A. Color matching, re-rasterizing to the proper dot density, and dithering to achieve light-to-dark graduations are all functions of the RIP. In a four-color printing process such as that shown in FIG. 1, the full dynamic range of the RGB image is converted to an equivalent dynamic range of the final CMYK image. The four color planes are then printed sequentially, as described supra.
This prior-art technology can be expanded to include eight colors, eight contrast controls, and eight ink applicators (not shown). In an eight-color printing process such as that in FIG. 2, the full dynamic range of the RGB image is converted to an equivalent dynamic range of the final cyan, cyan-like, magenta, magenta-like, yellow, yellow-like, black, and black-like (or gray) colors. The eight color planes are then printed sequentially, as described supra. The prior-ail, eight-color printing process requires four extra colors of ink and the apparatus to apply them to the final image surface. This is costly and requires extra quality control steps to ensure the ink colors are correct.